北斗三号系统卫星自主完好性监测技术

完好性对于全球卫星导航系统(GNSS)来说至关重要,关乎到其能否被放心应用。卫星自主完好性监测(SAIM)技术,是完好性监测技术发展的前沿趋势,国内外卫星导航系统均竞相发展。介绍了北斗三号系统SAIM技术设计与实现的重要意义,从功能设计和实现原理两个方面,阐述了北斗三号SAIM技术体制。针对SAIM实际在轨监测数据的正态分布特性服从程度和长期稳定性等问题,随机选取某一颗中圆轨道(MEO)卫星自2020年7月31日北斗三号系统正式开通服务以来至2021年7月31日连续1年期间的监测数据,得到真实在轨监测数据的分布特性。最后,提出了告警门限优化、分级告警策略设计、星历完好性自主监测等方面的后续发展必要性建议,旨在为北斗系统更好地为全球用户提供更优质的完好性服务提供参考。     

BeiDou-3 orbit and clock quality of the IGS Multi-GNSS Pilot Project

Within the Multi-GNSS Pilot Project (MGEX) of the International GNSS Service (IGS), precise orbit and clock products for the BeiDou-3 global navigation satellite system (BDS-3) are routinely generated by a total of five analysis centers. The processing standards and specific properties of the individual products are reviewed and the BDS-3 orbit and clock product performance is assessed through direct inter-comparison, satellite laser ranging (SLR) residuals, clock stability analysis, and precise point positioning solutions. The orbit consistency evaluated by the signal-in-space range error is on the level of 4–8 cm for the medium Earth orbit satellites whereas SLR residuals have RMS values between 3 and 9 cm. The clock analysis reveals sytematic effects related to the elevation of the Sun above the orbital plane for all ACs pointing to deficiencies in solar radiation pressure modeling. Nevertheless, precise point positioning with the BDS-3 MGEX orbit and clock products results in 3D RMS values between 7 and 8 mm.     

Tropospheric wet refractivity tomography using multiplicative algebraic reconstruction technique

Algebraic reconstruction techniques (ART) have been successfully used to reconstruct the total electron content (TEC) of the ionosphere and in recent years be tentatively used in tropospheric wet refractivity and water vapor tomography in the ground-based GNSS technology. The previous research on ART used in tropospheric water vapor tomography focused on the convergence and relaxation parameters for various algebraic reconstruction techniques and rarely discussed the impact of Gaussian constraints and initial field on the iteration results. The existing accuracy evaluation parameters calculated from slant wet delay can only evaluate the resultant precision of the voxels penetrated by slant paths and cannot evaluate that of the voxels not penetrated by any slant path. The paper proposes two new statistical parameters Bias and RMS, calculated from wet refractivity of the total voxels, to improve the deficiencies of existing evaluation parameters and then discusses the effect of the Gaussian constraints and initial field on the convergence and tomography results in multiplicative algebraic reconstruction technique (MART) to reconstruct the 4D tropospheric wet refractivity field using simulation method.     

Near-real time monitoring of the TEC fluctuations over the northern hemisphere using GNSS permanent networks

Since the early 1990s, global positioning system measurements have been used to study of the state and rapid changes of the Total Electron Content in the ionosphere. Currently, the increasing number of permanent stations makes it possible to generate maps of the irregularities in the ionosphere for specified regions with sub-daily resolution. The main goal of this work was to apply global navigation satellite system observations to obtain information about ionospheric variability around the North Geomagnetic Pole. In order to detect the ionospheric disturbances, 30-s observation data was used. The Rate of Total Electron Content Index was applied as a measure of the variability in the ionosphere. The first analyses were executed using more than 100 permanent stations. The results show two kinds of products: 2-hour maps in spherical geomagnetic coordinates and daily maps presenting the occurrence of the strong Total Electron Content fluctuations as a magnetic local time function, for the most disturbed days of April 2010. Apart from the main product of the algorithm, the Rate of Total Electron Content time series for individual satellite tracks was presented. The results demonstrated very good sensitivity of the obtained maps, which can detect even quite weak disturbances. The presented algorithm developed at the Geodynamic Research Laboratory of the University of Warmia and Mazury, in cooperation with Institute of Terrestrial Magnetism, Ionosphere and Radiowave Propagation, will be applied in the near future to create near-real time service of the conditions in the ionosphere based on the Global Navigation Satellite Systems observations.     

GNSS Satellite Autonomous Integrity Monitoring (SAIM) using inter-satellite measurements

Integrity is the ability of Global Navigation Satellite Systems (GNSS) to detect faults in measurements and provide timely warnings to users and operators when the navigation system cannot meet the defined performance standards, which is of great importance for safety of life critical applications. Compared with both Receiver Autonomous Integrity Monitoring (RAIM) and ground based GNSS Integrity Channel (GIC) methods which are widely adopted nowadays, the Satellite Autonomous Integrity Monitoring (SAIM) method can be used to monitor orbit/ephemeris and clock errors, and has advantages in monitoring orbit and clock quality and providing instantaneous responses when faults happen.     

数字域谱估计的GNSS系统信号带宽测试方法

针对传统的单载波扫频法不适于GNSS（全球导航卫星系统）信号生成通道的带宽测量的问题,提出了采用数字信号处理的信号带宽方法.对实际导航信号进行高速采样,在数字域对采样的导航信号和本地数字生成的理想导航信号进行频谱估计和计算,从而得到GNSS信号带宽.分别使用周期图法和Welch法谱估计,对提出的测试方法进行仿真.仿真结果表明,采用Welch法进行理想信号和实际信号谱估计,对谱估计后的结果进行计算,可以完成GNSS信号带宽精密测量,在Welch法估计参数设置合理的情况下,GNSS信号带宽估计的准确度可以达到99.9％以上.     

基于矢量跟踪的MEMS-SINS/GNSS深组合导航系统

针对在载体高动态运动时,由Doppler频移剧烈变化而导致的MEMS-SINS/GNSS组合导航系统精度降低的问题,修正了GNSS卫星信号模型,设计了一种深组合导航通道滤波器,并形成了完整的深组合导航方案。基于矢量跟踪结构的深组合导航方案,通过滤波器对载体的导航参数进行估计,将相互独立的各通道信息结合起来,并利用各卫星位置之间的关系,提高了高动态环境中的跟踪能力。仿真结果表明,基于矢量跟踪结构的深组合导航方案可以有效减少高动态对导航精度的影响,提高组合导航系统的输出精度。     

GNSS接收机在探月三期中的应用研究

探月工程对飞行器导航系统的性能提出了更高要求,因而有必要研究GNSS(Global Navigation Satellite System,全球导航卫星系统)接收机在探月任务中的应用。以探月工程三期为应用需求,开展地-月-地转移过程中的GNSS接收机可用性分析、GNSS信号特性分析;并针对低信噪比环境下的高灵敏度接收机系统应用进行设计,采用弱信号捕获算法和弱信号跟踪算法实现-175dBW的灵敏度;最终采用基于轨道动力学模型的卡尔曼滤波方法实现了GNSS自主定轨算法。仿真表明:GNSS接收机在60 000km以下的地-月转移和月-地转移弧段能够为探月飞行器提供100m以内位置精度的导航服务。     

IGS电离层产品在双向时间频率传递中的应用

利用IGS组织提供的全球电离层资料对卫星双向时间频率传递中的电离层误差进行修正。IGS提供特定时刻、固定经纬度网格点上的电离层总电子含量。对该电离层资料首先进行空间四点网格内插,然后利用双线性内插得到电离层穿刺点所需时刻的总电子含量,最后将得到电离层数据经过处理用于双向时间频率传递修正。结果表明：电离层对C波段的影响在（0～0.5）ns范围内,这对亚纳秒量级的时间比对是必须考虑的。IGS提供的电离层产品适合应用于双向时间频率传递,具有方法简单、准确度高和价格低廉等特点。